docs: def ifndefs():
      """generate `#ifndef ZMQ_CONST` block for zmq_constants.h"""
      lines = ['#define _PYZMQ_UNDEFINED (-9999)']
      for name in all_names:
          if not no_prefix(name):
              name = 'ZMQ_%s' % name
          lines.append(ifndef_t.format(name))
      return dict(ZMQ_IFNDEFS='\n'.join(lines))